3CX SBC Drawbacks

The benefits of the 3CX SBC are well-advertised and well-documented - I'm a huge fan of it myself. But I was unable to find a list of disadvantages to using an SBC as opposed to an on-prem solution.
From what I've seen so far, third-party phones can potentially lose a lot of functionality with SBC's, visibility and monitoring for SBC connections are still in the works, and another example is the Hotdesking feature not being available without a local PBX.
In what other scenarios would an on-prem PBX solution be preferable to an on-prem SBC?

I would prefer using VLAN and/or VPN topology instead, if possible, and you have suitable routers and control over them.
SBC is a great technology, if you don't have control over network infrastructure.

3CX Advanced Certified Engineer
+44 8444100333
info (at) sip.bg

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+1 for what sip.bg said. If the client has the correct network infrastructure and internet to support VoIP, hopefully with some redundancy, then local VLAN plus VPN for remote locations works very well. But, for small operations without these the SBC works great if you stick with recommended phones.

What makes a VPN setup superior to an SBC setup? I thought the SBC provided benefits that VPN's can't.

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Depends whether you can build a VPN or not. If possible you shouldn't have limitation on number of phones used in remote locaton, while depending on hardware of SBC, you have. Second: you may use any phone that works in a local network with 3cx plus fax ATA, not the case with SBC. Third: You may implement QoS in your router for VoIP, not the case with SBC. Next: You may have backup routes for VPN, backup ISP, BGP, etc. Fifth: VPN adds encryption, typically IPsec with high level of security nowadays. SBC is generally equivalent to using GRE tunnel without encryption. Next: SBC is additional device and another point of failure, you still have routers in place.

I have several customers with multiple locations and between 500 and 1000 remote extensions. I would never use SBCs in such scenario.

This is not the case if you don't have control over routers, or they are not supporting VPN or you live in country where using VPN is forbidden. With some 3CX cloud implementations is not possible to use VPN. Probably for some users will be easier to set SBC than VPN also. SBC is a proprietary 3CX solution, you may get support for it, while with VPN you must rely on your own. Configuring VPN in some routers can be complicated. So, it depends on your infrastructure. In most cases you can still use STUN for your remote phones, regardless it is less secure than using SBC. SBC provides some abstract level of independence from your network/router infrastructure.

3CX Advanced Certified Engineer
+44 8444100333
info (at) sip.bg

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We have been using the 3cx SBC for years and throughout it all it has provided service to our clients without request from them for more functionality whether on raspberry pi or windows it does just what it needs to do. Incrementally 3cx has added features to make it more robust and fault tolerant all-in all it just works with out the extra areas of disturbance with vpns and vlans. Our larger installations run 60 plus handsets over two locations to the cloud. Its a small price to pay to have to use supported phones and how is that any different from other vendors like cisco, Avaya, or Toshiba...